Refiner network controller

ABSTRACT

The ratio of refining amongst plural fiber stock refining branches is maintained substantially constant by maintaining a substantially constant, predetermined stock flow ratio amongst the branches and by maintaining substantially constant the ratio of work per unit weight of fibers exerted by the refiners of the plural branches on the fibers. The flow ratio is maintained substantially constant by effectively measuring the flow in each of the branches, responding to the measurements to derive an indication of the flow ratio for each branch relative to the total flow, and comparing the indicated flow ratios with flow ratio setpoints to actuate valves in branches auxiliary to a main branch. The ratio of net work per unit weight of fibers is maintained substantially constant by combining a signal indicative of the flow through each individual branch with a signal indicative of the net power ratio of each branch relative to the main branch to derive an indication of the power to be consumed by the individual branch. The indication of power to be consumed by each branch controls the clearance between cutting members of each refiner of the auxiliary branches.

Tl'nite States atent 1.191

Spitz July 9, 1974 REFINER NETWORK CONTROLLER ing branches is maintained substantially constant by maintaining a substantially constant, predetermined [75] David Spitz Columbus Ohio stock flow ratio amongst the branches and by main- [73] Ass1gnee: Industrial Nucleonics Corporation, taining substantially constant the ratio of work per Columbus, Ohio unit weight of fibers exerted by the refiners of the plural branches on the fibers. The flow ratio is main- [22] Flled' 1973 tained substantially constant by effectively measuring [2]] Appl. No.: 340,664 the flow in each of the branches, responding to the measurements to derive an indication of the flow ratio 52 us. c1. 241/33, 241 /37 dame P f [51 1 Int Cl 302C 25/00 mg the indicated flow ratios wlth flow ratio setpomts to actuate valves in branches auxiliary to a main [58] Field of Search 241/33, 34, 35, 36, 37,

162/252, 253, 254, 258 branch. The ratio of net work per unit we1ght of fibers is maintained substantially constant by combining a [561 References Cited fir aifilfi'lfiii fi i fiififdfiiiivi ififi 323331! 52253 UNITED STATES PATENTS of each branch relative to the main branch to derive 3,568,939 3/i97i Bl'eWStfif et ai 241/37 X an indication of the power to be onsumed the ing dividual branch. The indication of power to be conumme e a 3,711,687 1/1973 Stout et al. l62/258 x Sumed by each branch Controls the clearance between Primary ExaminerGranville Y. Custer, Jr.

[ 5 7 ABSTRACT .The ratio of refining amongst plural fiber stock refin- FROM 43 CONTROLLER '54 HNZDWOOD cutting members of each refiner of the auxil iar branches.

7 Claims, 2 Drawing Figures FR M 43 CONTDLLER 3B '13 42 1 COMPUTER l I I l 1'! To CONTROLLERS lliii-i MAG-NE MENTEDJUL SHEET 1 BF 2 F/& f

FROM 42 TO 4% R f CONTROLLERL/BA 47.1 W F3 4| 3% HAROWOOO -& i 132 \158 I L FROM A3 CONTROLLER 33 42 F PME CbMPUTER TO CONTROLLEQS CONTROLLER MACH M 3 E CHEST PATENIEUJUL 91914. 3,822,828

SHEET 2 OF 2 76. 2 ATE, 55 x viii: ISTo comm TOCONIB?) AT I V l 5Q, L J

f TO CONT. 48

$2 N Ngfl F3 \NC5 W3 3\.

DET. V ALNZM 1L Wu L TWO CONT. 45

T0 com. 4%

DET. ALARM REFINER NETWORK CONTROLLER supplying stock to fiber sheet producing machines, and

more particularly, to controllers for networks of plural refiners.

BACKGROUND OF THE INVENTION Refiner networks feeding stock to fiber sheet producing machines, such as papermaking machines, frequently include a number of branches which feed stock to a common outlet thence to the machine. The refiners of the different branches usually have differing characteristics with regard to the severity with which they are capable of fibrillating the fibers. This is evident since one network may include refiners of an entirely different nature, whereby certain refiners may be taperedplug refiners and others disc refiners. Further, there are material differences in characteristics of re finers of the same type. The problm is further frequently compounded since different branches of a network may be responsive to fibers having entirely different characteristics; one set of branches may be responsive to stock from a soft wood, such as pine, while a further branch or group of branches maybe responsive to a hard wood stock. the problem is further compounded because a refiner network is subject to considerable variation, with regard to the number of branches and the number of refiners in a particular branch. The number of branches and number of refiners in a branch may be varied depending upon the load requirements of a particular machine and the status of a particular refiner in the network; one refiner may be operative or inoperative at a particular time. All of these factors make it particularly difficult in a typical refining network to provide material that is in a common line being fed by several branches substantially consistent as a function of time and homogeneous.

I am aware of prior work by others relating to the operation of parallel operated refiners as follows:

Brewster et al.; 3,568,939

Rummel et al.; 3,687,802

Stout et al.; 3,711,687 It is my opinion that these prior art systems do not provide the greatest possible consistency and homogeneity of stock flowing from a common outlet of the plural refiner branches to the machine. In addition, the systems disclosed by Stout et al and Rummel et al require a measurement of consistency of fiber flowing from a single stock supply to the refiner network, thereby obviating the ability to respond to stocks from different sources. The consistency measurement is also difficult and expensive to provide. Further, the prior art systems involve computer operations which appear to require a relatively large amount of memory space in a general purpose digital computer.

BRIEF DESCRIPTION OF THE INVENTION In accordance with the present invention, a new and improved controller for a refiner network is provided by maintaining a substantially constant, predetermined stock flow ratio amongst the different branches and by maintaining substantially constant the ratio of work per unit weight of fibers exerted by the refiners of the branches on the fibers passing through the branches. A

refiner network controlled in accordance with the present invention provides stock flowing from a common outlet of the parallel branches to the papermaking machine that is relatively consistent asa function of time and homogeneous, since the refining ratio of the several branches is maintained constant for varying conditions.

By maintaining a substantially constant flow ratio amongst the branches, the severity introduced by each of the branches on the stock flowing therein is maintained at a predetermined ratio. For branches responsive to sources of different stock types, the severity of the differing branches is generally not the same if the individual branches are to provide stock having substantially the same fibrillation. It is desirable with the flow ratio control of the present invention to provide power ratio control amongst the branches, rather than absolute power control amongst the branches, because the flow through a particular branch governs the amount of work performed by the branch on the fibers.

'If there wereno power ratio control as a function of flow, fibrillation in a particular branch could not be maintained at a predetermined level.

The use of a power ratio control has the further advantage, with or without the severity control achieved by flow control, of obviating the requirement to measure consistency of stock flowing to the refiner network and of generally simplifying many of the computations. These features arise because like terms in the numerator and denominator of a ratio cancel each other and since a network control computer use with the invention handles ratios rather than absolute values in many instances.

It is, accordingly, an object of the present invention to provide a new and improved refiner network controller.

Another object of the invention is to provide a new and improved controller for providing equal refining in a plurality of branches feeding a fiber sheet making machine through a common outlet.

Another object of the invention is to provide a new and improved refiner network controller capable of delivering stock from several branches to a common outlet so that stock in the outletis relatively consistent as a function of time and homogeneous, even though the stock comes from differing sources.

Another object of the invention is to provide a new and improved refiner network controller which does not require a consistency measuring device.

Still another object of the invention is to provide a new and improved refiner network controller wherein the net work per ton in individual branches is maintained at a substantially constant ratio despite variations in flow induced in the individual branches to control severity of refining therein.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a schematic diagram of a typical refiner network controlled in accordance with the present invention; and

FIG. 2 is a block diagram of apparatus included in the computer of FIG. 1.

DETAILED DESCRIPTION OF THE DRAWING In FIG. 1 there is illustrated a refiner network including branches 11, 12 and 13, which feed a fiber sheet producing machine (not shown) through a common outlet conduit 14. Branches l1 and 12 are connected in parallel to a single source of pine fibers while branch 11 is responsive to a hard wood fiber source. Each of branches 12 and 13 includes a single refiner l7 and 18, while branch 11 includes series refiners l and 16. The refiner network can be varied from the configuration illustrated in FIG. 1, depending upon the type of sheet being manufactured, the operation characteristics of the individual refiners, as well as the requirements of the machine. The network of FIG. 1 is provided solely for purposes of disclosing an exemplary system.

Branch 11 is considered as a main branch, through which the greatest percentage of fiber generally flows to the machine, while branches l2 and 13 are auxiliary branches. The power and flow rates of main branch 11 are considered as references to which the power and flow rates of branches 12 and 13 are ratioed.

The total flow through the network is controlled by valve 21 in conduit 14 in response to the level of stock in machine chest 22. The stock level in machine chest 22 is preferably maintained at a predetermined level by providing the machine chest with a level detector 23 which derives an output signal that is compared with a level setpoint in difference network 24. The error signal derived from difference network 24 is applied to integral controller 25 which generates an output signal to drive valve actuator 26 for valve 21. Thereby, as the stock level in machine chest 22 rises and falls, valve 21 respectively closes and opens to control the total flow through conduit 14. This total flow control is old and well known to those skilled in the art.

To maintain substantially constant the severity ratio amongst the auxiliary branches 12 and 13 relative to the refining of main branch 11, the auxiliary branches are provided with flow control valves 31 and 32. No valve is provided in main branch 11, but the flow in the main branch is governed by the setting of valve 21, as well as by the flow rates of branches 12 and 13 into conduit 14. Valves 31 and 32 are provided with integral controllers 33 and 34 which derive signals for valve actuators 35 and 36 of valves 31 and 32.

Flow controllers 33 and 34 are responsive to setpoint values for the flow ratio through branches l2 and 13 relative to branch 11, as well as to signals indicative of the actual flow ratio of the auxiliary branches to the main branch, as derived by monitoring flow through branches 11, 12 and 13. The flow rate measurements of branches 11, 12 and 13 can be provided directly by utilizing magnetic flow meters or inferentially by measuring the change in stock temperature in a branch, as well as the power consumed by the refiners of the branches. To measure the change in temperature in each branch, each branch is provided with a pair of temperature transducers 37 and 38 respectively upstream and downstream of the first and last refiner of the branch. The signals derived from temperature transducers 37 and 38 are supplied to a different subtraction network 39 for each of the branches. The power dissipated by each of the refiners is monitored by measuring with watt meters 42 the power consumed by constant speed motors 41 which rotatably drive a cutting element of each of refiners 15-18. To derive the flow indications forbranches 12 and 13, the measurements of the watt meters for these two channels are divided by the temperature difference measurements for these branches. In branches having more than one refiner, i.e., branch 11, it is assumed that the power of each refiner is equal, whereby the power measurement of watt meter 42 for refiner 15 is multiplied by the number of refiners in the branch to determine the total branch power. To determine flow, the product is divided by the temperature difference signal for the branch. The signals derived from difference networks 39 and from watt meters 42 are supplied to computer 43 which derives output signals for activation of flow controllers 33 and 34, as well as to control the power exerted by the different refiners on the stock flowing therein.

Power control for the different refiners is responsive to an indication of the flow through the refiner, as well as desired net work per unit weight power ratios of the auxiliary branches 12 and 13 relative to main branch 11. Power control of the different refiners is achieved by varying the clearance between adjacent cutting elements including in the refiners. To this end, each of the refiners includes a motor 44 for translating one refiner cutting element relative to another element. The motors 44 for refiners 15, 16, 17 and 18 are respectively responsive to integral controllers 45, 46, 47 and 48.

Consideration is now given to FIG. 2 of the drawing wherein there is illustrated in block diagram form the computer 43 of FIG. 1. The computer 43 includes flow controller channels 52 and 53 which are respectively provided for flow controllers 33 and 34, as well as power controller channels 54 and 55 which are provided for the power controllers 47 and 48 of branches 12 and 13. The flow control channels 52 and 53 are quite similar, except for inputs, as are the power control channels 54 and 55. Power control channel 56 for controllers 45 and 46 of main branch 11 is also provided, but differs materially from the power control channels 54 and 55.

For the following description of computer 43, subscripts denote parameters of the different refiner channels and refiners within the channels; parameters of branches 12 and 13 are provided with the subscripts 2 and 3, respectively, while common parameters of branch 11 are provided with the subscript 1; parameters for individual refiners 15 and 16 of branch 11 are respectively provided with the subscripts 11 and 12. Power supplied to and dissipated by a refiner are respectively indicated by P and W, while branch flow rate and temperature difference are indicated by F and AT.

To indicate the flow rates (F and F of branches 12 and 13, the power indications (W and W for refiners 17 and 18 are respectively divided by the branch temperature difference (AT and AT;,) signals in dividers 56 and 57. The flow rate of branch 11 is determined by multiplying the power indication of refiner 15 (W,,) by the number (N) of refiners in the branch, where N 2, based on the assumption that the power is divided equally amongst the plural refiners of the branch. Thereby, the power of refiner 15 (W,,) is multiplied by two in multiplier 58, which derives an output signal commensurate with the total power consumed in branch 11. The total power output signal of multiplier 58 is divided by the temperature difference across branch 11, AT in divider 59. To determine total flow in the refiner network, the output signals of dividers 56, 57 and 59 are added together in summing network 61. The frictional flows in branches 12 and 13 are determined by supplying the output signal of network 61 in parallel to channels 52 and 53, where it is combined with the flow indications for branches 12 and 13.

It is also necessary to determine the fractional flow setpoints (F IF and F /F where: F and F are setpoints for the flows in branches 12 and 13 relative to main branch 11 and F is a computed total flow setpoint of the network relative to branch 11 for channels 12 and 13 relative to total flow. To thisend, operator controlled input setpoint signals indicative of the percentage of desired flows (F and F in branches 12 and 13 relative to the flow through branch 11 are respectively derived from sources 62 and 63. A further source 64 has a predetermined value, equal to one hundred for the percentage of the flow of branch 11 relative to itself. The output signals of sources 62, 63 and 64 are added together in summing network 65, which derives an output signal indicative of the total percent flow of the network relative to main branch 11. The output signal of summing network 65 is applied in parallel to channels 52 and 53, where it is combined with signals from sources 62 and 63 to derive the friction flow setpoints of branches 12 and 13 relative to total refiner flow.

' Considering channel 52 in detail, there is included a division network 66 responsive to the output of summing network 61 as well as the flow indicating signal derived from divider 56. Network 66 generates an output signal indicative of the friction of the flow in branch 12 relative to the total flow as actually measured, i.e., F /F F, F The actual branch 12 output signal of divider 66 is compared in subtraction network 67 with an indication of the branch 12 setpoint fractional flow, F /F as derived by divider network 68. To this end, divider network 68 is responsive to the output signal of summing network 65, as well as a signal indicative of the setpoint for the percentage flow in branch 12, as derived from source 62, i.e., divider 68 derives an output signal in accordance with F 100 F F Subtraction circuit 67 responds to the two input signals thereof to derive an error signal, indicative of the deviation of the actual flow ratio of branch 12 from the setpoint flow ratio for branch 12. The output signal of subtraction network 67 is applied to controller 33 to vary the setting of valve 31 and thereby the flow in branch 12. Similarly, channel 53 responds to the output signals of dividers 57 and 65 and the setpoint signal for the percent flow rate of branch 13 to derive an error signal flow rate through branch all of the refiners of branch 11 are substantially the same and by subtracting a predetermined signal indicative of the power (P consumed by one of the refiners,

6 e .g., refiner 15, when it is in an idle condition, i.e., the amount of power consumed by the refiner when the cutting elements thereof are separated by so great a distance as to prevent fibrillation of fibers flowing between them. The value of P is determined for each refiner by utilizing known techniques and is manually entered into the system, as a predetermined value. The operator also enters into computer 43 a predetermined signal indicative of the setpoint for the power (P consumed by one of the main branch refiners, e.g., refiner 15. The value of P is subtracted from P, in subtraction network 71, which derives an output signal indicative of P P The output signal of network 71 is thereby indicative of the acual, net power exerted by refiner 15 on the stock flowing through main branch 11 and is supplied in parallel to channels 54 and 55.

Channels 54 and 55 are also responsive to operator controlled input signals indicative of: the number (N,) of refiners in the main branch, two in the present instance; the setpoints for the net work per unit weight of each particular branch as a fraction of the net work per unit weight of the main. branch, (R and R the number of refiners in the particular branch (N and N both equal to one in the present instance); the no-load power of the refiners of branches l2 and 13 (W and W the actual power consumed by the refiners of branches 12 and 13 (W and W and the power limit for refiners l7 and 18 (W and W Channels 54 and 55 are also responsive to signals indicative of the actual flow rates through branches 12 and 13. In the embodiment of FIG. 2, the flow rates are set equal to the setpoint flows F and P but it is to be understood that an actual percentage flow rate indicating signal can be provided, if desired. However, the use of the F and F signals has the advantage of providing ratio control relative to flow in the main branch, without further calculation.

Consideration is now given to the specific apparatus of channel 54 wherein a signal is derived to activate controller 47 for the clearance of refiner 17 and thereby control the refiner power. A setpoint for the net power of refiner 17 is derived by multiplying the net power indicating setpoint signal of one refiner of the main channel, as derived from difference network 71, by an operator predetermined setpoint (R signal for the net work per unit weight for branch 12 relative to the main branch; the multiplication operation is performed in multiplier 73. The operator therefore does not set the value of the power for the auxiliary branch 12, but merely decides on the percentage of net power to be exerted by the auxiliary branch refiners on the fiber relative to the net power to be exerted by the main branch refiners on the fibers. The computer then determines what the actual power requirements are for the auxiliary branch refiners. To this end, the output signal of multiplier 73 is multiplied by two since there are twice as many refiners in the main branch as in branch 12. The factor of two is formed in division network 74, responsive to the N and N signals, and the output of division network 74 is applied to multiplier 75. The output signal of multiplier 75 is combined in multiplier 76 with a signal indicative of the flow rate of branch 12 relative to the main branch flow rate per unit weight of fiber. To this end, divider 77 responds to a percentage fiow rate signal (F for branch 12, as derived from source 62, and a predetermined signal having a value of one hundred for the percentage flow through the main branch. The output of divider 77 (F 100) is fed to multiplier 76 which derives an output signal indicative of net power of a refiner in branch 12.

If branch 12 included several series refiners, the output signal of multiplier 76 would be applied in parallel to a number of networks each of which would derive a separate control signal for the individual series refiners of the branch. However, since only one refiner is included in branch 12, there is only one individual refiner control network provided in channel 54.

The refiner control network of channel 54 includes a suming network 78 responsive to the no-load power of refiner 17 (W and the net power output of multiplier 76. Summing network 78 thereby derives an output signal indicative of a setpoint for the total or gross power required of refiner 17. To control the power consumed by refiner 17, the output signal of summing network 78 is applied to comparison network 78 from a measurement of the power (W actually being consumed by refiner 17, as mentioned by watt meter 42 for motor 41 of the refiner. Subtraction network 79 derives an error signal that is fed to integral controller 47 which drives motor 44 to control the clearance between cutting elements of refiner l7.

In the event that refiner 17 is unable to handle the refining load defined by the setpoint signal derived from summing network 78, an alarm is activated to apprise the operator that he should connect an additional refiner in series with refiner 17. To this end, the gross power output signal of summing network 78 is subtracted from a predetermined limit signal for the gross power (W of refiner 17 in comparison network 81. In response to the output signal of comparison network 81 exceeding a predetermined level, detector 82 is activated to energize an alarm 83 which may be in the form of any or all of: a print-out to indicate refiner 17 having an excessive load demand, an aural indication, and a visual indication.

Channel 55 includes substantially the same apparatus as is disclosed in conjunction with channel 54 and derives control and alarm signals for branch 13 in a similar manner.

Control of refiners l5 and 16 of main branch 11 is based upon a slightly different theory relative to the theory for control of the refiners of branches 12 and 13. This is because the friction of the setpoint for branch 11 to the main branch is one. Since the main branch includes a pair of serial refiners 15 and 16, the power setpoint signal (P for one refiner of main branch 11 is applied in parallel to separate networks 84 and 85, one of which is provided for each of refiners l5 and 16.

Considering channel 84, the power setpoint signal for one main branch refiner (P is applied to one input of comparison network 87, where it is combined with the measured power of refiner 15 to derive an error signal (P -W for activating integral controller 45. The output of comparison network 87 is also applied to an alarm activating system as discussed supra with regard to channel 54 but which is responsive to a predetermined quantity indicative of the power limit for refiner 15. Channel 85 is substantially the same as channel 84, but includes inputs responsive to the actual power being consumed by refiner 16, and the power limit of refiner 16, to derive output signals for controller 46 and actuation of an alarm.

While there has been described and illustrated one specific embodiment of the invention, it will be clear that variations in the details of the embodiment specifically illustrated and described may be made without departing from the true spirit and scope of the invention as defined in the appended claims. For example, the principles of the invention are applicable to systems where it is not assumed that the power is equally divided amongst serial refiners of a single branch. If it is assumed that the power is not equally divided amongst the everal refiners of a single branch, but that the separate refiners have different characteristics, the power ratio of the refiners in a branch is determined and apportioned in a manner somewhat similar to that for which the power is ratioed amongst the several branches. Also, the value of P supplied to node 71 can be derived by feedback means from a measurement made on the machine and indicative of sheet strength. The measurement can be derived, e.g., by monitoring freeness on a Fourdrinier wire, viscosity of fluid in line 14, couch vacuum, or flat box vacuum. While the principles of the invention have been disclosed in conjunction with a special purpose computer generally of the analog type, it is to be understood that the refiner control system of the invention could include a digital computer of the special or general purpose type. I

I claim:

1. Apparatus for maintaining substantially constant the ratio of refining amongst plural fibrous stock refining branches comprising means for maintaining a substantially constant predetermined stock flow ratio amongst the branches, and means responsive to an indication of flow in the individual branches for maintaining substantially constant the ratio of work per unit weight of fibers exerted by the refiners of the plural branches on the fibers.

2. The apparatus of claim 1 wherein the means for maintaining the ratio of work per unit weight of fibers includes a signal source for indicating desired power in auxiliary ones of said branches relative to a main branch.

3. Apparatus for maintaining substantially constant the ratio of refining amongst plural (N) fibrous stock refiner branches comprising: means, including measuring means, for indicating the ratio of flow in at least (N-l) of said branches to the total flow in said branches, means responsive to each of the ratios for controlling the flow in each of said at least (N-l) branches, and means responsive to an indication of flow in the at least (N-l) branches for maintaining substantially constant the ratio of work per unit weight of fibers exerted by the refiners of the plural branches on the fibers.

4. The apparatus of claim 3 wherein the means for maintaining the ratio of work per unit weight of fibers includes a signal source for indicating desired power in the at least (N-l) branches relative to the other branch.

5. Apparatus for maintaining a substantially constant predetermined power ratio amongst a plurality (N) of fibrous stock refiners comprising means for deriving a first signal indicative of the flow through at least (N-l) of said refiners, means for deriving a second signal indicative of the power to be exerted by one of the refiners on a unit weight of fibers relative to the power to be exerted by another of the refiners, means responsive to the first and second signals for the at least (N-l) refiners for deriving an indication of the power to be consumed by the at least (N-l) refiners, and means responsive to the indication for controlling the clearance between cutting members of the at least (N-l) refiners.

6. Apparatus for maintaining a substantially constant predetermined power ratio amongst a plurality (N) of fibrous stock refining branches feeding a common outlet comprising means for deriving a first signal indicative of the flow through at least (N-l) of said branches, means for deriving a second signal indicative of the power to be exerted by one of the branches on a unit weight of fibers relative to the power to be exerted by another of the branches, means responsive to the first and second signals for the at least (N-l) branches for deriving an indication of the power to be consumed by the at least (N-l) branches, and means responsive to the indication for controlling the clearance between cutting members of the refiners of the at least (N-l) branches.

7. Apparatus for controlling refining in a plurality (N) of different refiner branches feeding a fibrous sheet making machine through a common outlet, each of said refiners including a fiber cutting element having variable clearance relative to a further element, comprising transducer means for deriving a signal indicative of fluid flow rate in each branch, means responsive to the fluid flow rate control signal for at least (N-l) of the branches, means in each of the at least (N-l) branches for controlling the fiber flow rate in the at least (N-l) branches in response to the flow rate control signal for the respective branch, and means for controlling the clearance of the refiners of the at least (N-l) branches in response to the signal indicative of the flow through the respective branch and an indication of desired ratio of power consumed by the refiners for the branch to the power consumed by the other branch. 

1. Apparatus for maintaining substantially constant the ratio of refining amongst plural fibrous stock refining branches comprising means for maintaining a substantially constant predetermined stock flow ratio amongst the branches, and means responsive to an indication of flow in the individual branches for maintaining substantially constant the ratio of work per unit weight of fibers exerted by the refiners of the plural branches on the fibers.
 2. The apparatus of claim 1 wherein the means for Maintaining the ratio of work per unit weight of fibers includes a signal source for indicating desired power in auxiliary ones of said branches relative to a main branch.
 3. Apparatus for maintaining substantially constant the ratio of refining amongst plural (N) fibrous stock refiner branches comprising: means, including measuring means, for indicating the ratio of flow in at least (N-1) of said branches to the total flow in said branches, means responsive to each of the ratios for controlling the flow in each of said at least (N-1) branches, and means responsive to an indication of flow in the at least (N-1) branches for maintaining substantially constant the ratio of work per unit weight of fibers exerted by the refiners of the plural branches on the fibers.
 4. The apparatus of claim 3 wherein the means for maintaining the ratio of work per unit weight of fibers includes a signal source for indicating desired power in the at least (N-1) branches relative to the other branch.
 5. Apparatus for maintaining a substantially constant predetermined power ratio amongst a plurality (N) of fibrous stock refiners comprising means for deriving a first signal indicative of the flow through at least (N-1) of said refiners, means for deriving a second signal indicative of the power to be exerted by one of the refiners on a unit weight of fibers relative to the power to be exerted by another of the refiners, means responsive to the first and second signals for the at least (N-1) refiners for deriving an indication of the power to be consumed by the at least (N-1) refiners, and means responsive to the indication for controlling the clearance between cutting members of the at least (N-1) refiners.
 6. Apparatus for maintaining a substantially constant predetermined power ratio amongst a plurality (N) of fibrous stock refining branches feeding a common outlet comprising means for deriving a first signal indicative of the flow through at least (N-1) of said branches, means for deriving a second signal indicative of the power to be exerted by one of the branches on a unit weight of fibers relative to the power to be exerted by another of the branches, means responsive to the first and second signals for the at least (N-1) branches for deriving an indication of the power to be consumed by the at least (N-1) branches, and means responsive to the indication for controlling the clearance between cutting members of the refiners of the at least (N-1) branches.
 7. Apparatus for controlling refining in a plurality (N) of different refiner branches feeding a fibrous sheet making machine through a common outlet, each of said refiners including a fiber cutting element having variable clearance relative to a further element, comprising transducer means for deriving a signal indicative of fluid flow rate in each branch, means responsive to the fluid flow rate control signal for at least (N-1) of the branches, means in each of the at least (N-1) branches for controlling the fiber flow rate in the at least (N-1) branches in response to the flow rate control signal for the respective branch, and means for controlling the clearance of the refiners of the at least (N-1) branches in response to the signal indicative of the flow through the respective branch and an indication of desired ratio of power consumed by the refiners for the branch to the power consumed by the other branch. 